Superalloy powder

ABSTRACT

The invention discloses a Ni— or Co-based superalloy powder enriched with at least one fusing element: B, such that each grain of powder includes said at least one fusing element distributed among the other elements of the superalloy. A powder of this kind already has the requisite final composition, both in terms of constituent elements of the superalloy and in terms of fusing element(s). In particular, the proportion of B and, optionally, Si is adapted for use of the powder without a preliminary step of mixing with another powder. Use of this powder to fabricate components, in particular plates, by sintering, or mixed with a cement, or as a constituent of a mixture for injection molding of metallic powders.

The object of the invention is a superalloy powder.

In the field of aeronautical engineering and industrial turbines, thesevere operating conditions imposed on certain components such asturbine blading and nozzles have dictated the need to manufacture thesecomponents in Ni— or Co-base superalloy. To assemble these components orto repair them by hardfacing (that is to say by localized deposition ofmaterial on the component), current techniques reliant on fusion weldingare found wanting or unsuitable for use. Also, as described in thedocument FR 2 822 741, brazing-diffusion methods have been developedusing so-called bi-component mixtures of two metallic powders. Thesemixtures include:

a first superalloy powder of chemical composition similar to that of thematerial to be repaired, and

a second powder of nickel (Ni) or cobalt (Co) base containing 2 to 6% byweight of fusing elements such as boron (B) or silicon (Si).

The presence of fusing elements in the second powder makes it possibleto lower the melting point of the latter and to work at a temperature atwhich the second powder is liquid, while the first powder remains in thesolid state.

These bi-component mixtures nevertheless have a number of drawbacks suchas the difficulty of making a uniform mixture of two powders, problemsof segregation of the powders during storage of the mixture, or problemsof proportioning each powder in the mixture. For example, when sinteringa bi-component mixture and the quantity of fusing element in certainregions of the mixture is not sufficient, a porous sintered material isobtained. Conversely, an excess of fusing element in certain regions ofthe mixture causes excessive fusion resulting in deformation of thesintered material, which then fails to conform to the requireddimensions.

To overcome these problems, a solution described in FR 2 822 741envisages a means of incrusting the grains of the second powder onto thegrains of the first powder by mechanical synthesis. In practice,however, this incrustation technique is found to be limited: thistechnique is found to be rather difficult to put into effect, inparticular by virtue of the fineness of the second powder used, whichcauses health problems. Moreover this technique only partially improvesthe homogeneity.

It is also known to chemically encapsulate the grains of the superalloypowder in layers of Ni—B and/or Ni—Si. From an industrial standpoint,this method is difficult to use in that it is very time-consuming andvery difficult to put into effect when the alloys are composed of aconsiderable number of elements in small proportions.

The purpose of the invention is to propose an alternative to theexisting solutions, offering good results in terms of uniformity ofdistribution of the fusing element(s) within the powder, which isreflected in particular by the absence of deformation of componentsfabricated by sintering.

To achieve this purpose, the object of the invention is a Ni— orCo-based superalloy powder according to claim 1 or claim 5.

It is not necessary to mix the powder of the invention with anotherpowder as in FR 2 822 741, as the powder of the invention already hasthe final composition that is needed, both in terms of constituentelements of the superalloy and in terms of fusing element(s). Inparticular, the proportion of B and, optionally, of Si is adapted forutilization of the powder without a step of preliminary mixing withanother powder (as explained above, the proportion of fusing elementshas a determining influence on the behavior of the powder during heattreatment of the latter).

In addition, in the powder of the invention, said fusing element formsan integral part of the superalloy: it is not chemically deposited ormechanically incrusted on the surface of the superalloy grains, as inthe known techniques referred to above.

In short, in the powder of the invention, the constituent elements ofthe superalloy, including the fusing element, are present in each grainof powder and are therefore distributed within the powder in a perfectlyhomogeneous manner. The problems of localized porosity and excessivefusion, associated with too low a proportion or too high a proportion offusing element in certain regions of the powder, are thus avoided.

Advantageously, to manufacture the powder of the invention, use is madeof a technique of atomizing a precursor liquid mixture, including theelements of said superalloy and said at least one fusing element.

The invention and its advantages will be better understood by readingthe detailed description that follows. This description makes referenceto the attached figures in which:

FIG. 1 is a photograph of a plate made by sintering from a powderaccording to the invention; and

FIG. 2 is a photograph of a plate made by sintering from a bi-componentmixture of powders.

Irrespective of the type of powder according to the invention, given byway of example below, each powder is a Ni— or Co-based superalloy powderwhich includes at least the three elements Ni, Co and Cr (chromium).

These powders were made using an atomization technique from a precursorliquid mixture including the elements of the superalloy (Ni, Co, Cr . .. ) and at least one fusing element (B and, optionally, Si). This liquidmixture was obtained by fusing alloys by induction, under vacuum, in acrucible equipped with a burette allowing the liquid mixture to flow outat a low rate of flow. Inert gas jets under high pressure, flowing at avelocity close to that of sound, are used to atomize the mixture leavingthe burette. The mixture then breaks down into fine droplets which thenassume a spheroidal shape under the effect of surface tension and cooldown very rapidly in an atomization chamber. In our case, the inertgases used are argon or nitrogen for example.

In a surprising manner, during cooling, there is no separation of thefusing element(s) from the other elements of the alloy. All of theseelements remain within each droplet, and therefore within each grain ofpowder.

Unless otherwise indicated, the percentages given below are percentagesby weight.

In a first type of superalloy powder according to the invention, basedon Ni, the superalloy enriched with fusing elements essentiallyincludes: 14 to 19.6% of Co; 8.2 to 15.3% of Cr; 2.6 to 4.7% of Mo; 2.25to 3.5% of Al; 1.95 to 3.1% of Ti; 0 to 2% of Si; 0.4 to 1.3% of B; anda remainder of Ni.

The presence of impurities in the powder is not ruled out (hence the useof the term “essentially”). For example, carbon (C), zirconium (Zr), andphosphorus (P) may be found in minimal proportions, for example in theorder of, or less than, 0.06%.

In a first example (a) of the first type of superalloy powder accordingto the invention, the superalloy enriched with fusing elementsessentially includes: 16.4 to 19.6% of Co; 8.2 to 12.8% of Cr; 2.6 to4.4% of Mo; 2.25 to 3.3% of Al; 1.95 to 2.9% of Ti; 0.8 to 2% of Si; 0.5to 1.3% of B; and a remainder of Ni.

In a second example (b) of the first type of superalloy powder accordingto the invention, the enriched superalloy essentially includes: 14 to16% of Co; 12 to 15.3% of Cr; 3.35 to 4.7% of Mo; 2.9 to 3.5% of Al; 2.5to 3.1% of Ti; 0.4 to 1% of B; and a remainder of Ni. In the example(b), B is the sole fusing element.

In a second type of superalloy powder according to the invention, basedon Co, the enriched superalloy essentially includes: 17.2 to 22.2% ofCr; 26.75 to 30% of Ni; 0 to 1.5% of Si; 0.8 to 1% of B; 0.1 to 0.5% ofC; 0 to 0.37% of Zr; 0 to 3% of Ta; and a remainder of Co.

The cobalt-base powders of the second type can include impurities, suchas phosphorus P, in minimal proportions, for example in the order of, orless than, 0.04%.

Table 1 below summarizes the compositions of the example powders (a) and(b), cited above, and of an example powder (c) corresponding to thesecond type of superalloy powder according to the invention. TABLE 1Composition in % by weight Ex Ni Co Cr Mo Al Ti Si B C Zr P Ta (a) base16.4 8.2 2.6 2.25 1.95 0.8 0.5 0 0 0 0 19.6 12.8 4.4 3.3 2.9 2 1.3 0.060.05 0.01 0 (b) base 14 12 3.35 2.9 2.5 0 0.4 0 0 0 0 16 15.3 4.7 3.53.1 0 1 0.06 0.06 0.02 0 (c) 26.75 base 17.2 0 0 0 0 0.8 0.1 0 0 — 3022.2 0 0 0 1.5 1 0.5 0.37 0.04 —

All of these examples of superalloy powder may be used in theimplementation of any brazing-diffusion method applied during themanufacture or repair of components made of nickel-based or cobalt-basedalloys, in particular in the field of aeronautical engineering. This caninclude assembly of components, filling of cracks, or fissures, on acomponent or hardfacing of the surface of a component with a view tocorrecting a superficial defect or restoring certain properties orgeometric dimensions of the latter.

Depending on the applications, placement of the filler powder can beeffected in different ways.

For the filling of cracks, the raw powder can be used mixed with acement, for example of the type Nicrobraz 320. It will be noted that themixture obtained can be applied in the form of beads.

In certain applications, and in particular in the case of hardfacing ofa component surface, the application can be effected in the form of acompact filler piece. Said compact filler piece is obtained from thepowder either by a fabrication technique imparting compaction bysintering the powder, or by techniques of injection molding of metallicpowders.

FIG. 1 shows an example of a compact filler piece made by sintering froma superalloy powder according to the invention. It takes the form of aplate designed to be used to build up the surface of a component.

This plate was made from a powder of the first type cited above,according to the following steps: kilning of the raw powder;distribution of the latter in a mold matching the dimensions andthickness of the desired sintered material; arrangement of the mold in afurnace to expose it to heat treatment. As an example of heat treatment,it is possible to apply (for a furnace pressure of 0.13 Pa) aprogressive temperature rise to 1,160° C., holding at this temperaturefor approximately 10 minutes, followed by gradual cooling.

For comparison, a compact filler piece was made by sintering from amixture of bi-component powder of known type. FIG. 2 shows the pieceobtained.

In practice, it was found that the invention made it possible todispense with the handling and storage of several different types ofpowders, and to avoid any powder mixing stage, which is critical from ahealth and safety perspective.

Furthermore, by using a single powder containing within each grain thecomposition necessary to produce a plate (that is to say all theelements of the superalloy and at least one fusing element), thesintering temperature is substantially reduced relative to thetemperature required for a bi-component mixture. By virtue of thehomogeneity of the powder and the reduction of sintering temperature, anotable improvement is obtained in the uniformity of the properties ofthe sintered plate and good preservation of the dimensions and shape ofthe latter, in particular retention of the desired dimensions and goodflatness.

Conversely, FIG. 2 illustrates the deformation problems that may beencountered during sintering of a bi-component mixture.

The homogeneity of the superalloy powder of the invention is alsoreflected in an improvement in the mechanical properties of the zonebuilt up using said plate.

In another example of use of the powder according to the invention, acompact filler piece can be made using known techniques of injectionmolding of metallic powder. These techniques generally make it possibleto obtain components of more complex shapes than those made by simplemolding followed by sintering.

To this end, the powder is mixed with a binder in a mixer. The binderincludes, for example, polypropylene, ethylene, vinyl acetate andparaffin. The mixing time must be such that plastification of themixture is obtained. The mixture is then cooled before being milled. Thegranular material thus obtained can be fed into the hopper of a pressand injected into molds of dimensions specific to the compact fillerpiece to be produced. The molded blank is then chemically stripped fromthe mold and said blank is sintered.

It was found that certain of the example powders in Table 1 were moreamenable to certain uses among the following uses a, b and c:

-   -   a) use of the powder mixed with a cement, to fill cracks for        example;    -   b) use of the powder to produce compact filler pieces, in        particular plates, by sintering; and    -   c) use of the powder as a constituent of a mixture for injection        molding of metallic powders.

The preferred uses of each example powder are indicated in Table 2below. TABLE 2 Example Use a Use b Use c (a) Yes Yes Yes (b) Yes Yes (c)Yes Yes

1. Superalloy powder, characterized in that the superalloy is enrichedwith at least one fusing element: B, such that each grain of powderincludes said at least one fusing element distributed among the otherelements of the superalloy and in that the superalloy is composed of, inweight percent: 14 to 24% of Co; 8.2 to 20% of Cr; 0 to 4.7% of Mo; 2.25to 8% of Al; 0 to 3.1% of Ti 0 to 3.3% of Si; 0 to 4.5% of Ta; 0 to 0.6%of Y; up to 1.3% of B; and a remainder of Ni.
 2. Superalloy powderaccording to claim 1, wherein said superalloy enriched with fusingelement is composed of, in weight percent: 14 to 19.6% of Co; 8.2 to15.3%ofCr; 2.6 to 4.7% of Mo; 2.25 to 3.5% of Al; 1.95 to 3.1% of Ti 0to 2% of Si; 0.4 to 1.3% of B ; and a remainder of Ni.
 3. Superalloypowder according to claim 2, wherein said superalloy enriched withfusing element is composed of, in weight percent: 16.4 to 19.6% of Co;8.2 to 12.8% ofCr; 2.6 to 4.4% of Mo; 2.25 to 3.3% of Al; 1.95 to 2.9%of Ti 0.8 to 2% of Si; 0.5 to 1.3% of B; et a remainder of Ni. 4.Superalloy powder according to claim 2, wherein said superalloy enrichedwith fusing element is composed of, in weight percent: 14 to 16% of Co;12 to 15.3% of Cr; 3.35 to 4.7% of Mo; 2.9 to 3.5% of Al 2.5 to 3.1% ofTi 0.4 to 1% of B; and a remainder of Ni.
 5. Superalloy powder,characterized in that the superalloy is enriched with at least onefusing element: B, such that each grain of powder includes said at leastone fusing element distributed among the other elements of thesuperalloy and in that the superalloy is composed of, in weight percent:17.2 to 23% of Cr; 26.75 to 32.4% of Ni; 0 to 2.5% of Si; 0 to 0.5% ofC; 0 to 0.4% of Zr; 0 to 3% of Ta ; 0 to 0.5% of Y; 0 to 8% d'Al; up to1.2% of B; and a remainder of Co.
 6. Superalloy powder according toclaim 5, wherein said superalloy enriched with fusing element iscomposed of, in weight percent: 17.2 to 22.2% of Cr; 26.75 to 30% of Ni;0 to 1.5% of Si; 0.8 to 1% of B; 0.1 to O.5% of C; 0 to 0.37% of Zr; 0to 3% of Ta; and a remainder Co.
 7. Superalloy powder according to anyone of claims 1 to 6, obtained by atomization of a liquid mixtureincluding the elements of said superalloy and said at least one fusingelement.
 8. Use of a superalloy powder according to any one of claims 1to 7 for the fabrication of components, in particular plates, bysintering.
 9. Use of a superalloy powder according to any one of claims1 to 7 mixed with a cement.
 10. Use of a superalloy powder according toany one of claims 1 to 7 as a constituent of a mixture for injectionmolding of metallic powders.